1. Field of the Invention
This invention relates generally to the field of identification and analysis of biological specimens, and more particularly to a method and system for identifying objects of interest, such as cancerous cells or cellular objects, in a biological specimen. The invention is also related to the field of color space transformations, in which a representation of an object in one color space (e.g., red, green and blue components) is transformed mathematically into a new representation in a new color space, in order to more easily observe or identify objects.
2. Description of Related Art
A biological specimen such as samples of bone marrow, cervical tissue, lymph nodes, or peripheral blood, may have objects of interest to a pathologist or histologist. Such samples are typically fixed to a slide and examined under a microscope. An important aspect of medical diagnostics is detecting, identifying, and quantitating the objects of interest within the biological specimen. The objects of interest may be, for example, cancer cells, cell objects such as nuclei, or particular proteins or clusters of proteins present in the biological specimen. The cancer cells or the particular proteins in the biological specimen can be difficult to detect. However, by staining the biological specimen with a stain, the objects of interest in the biological specimen can be made more readily identifiable.
A staining process involves introducing a probe that is reactive with a component of the objects of interest. The probe typically is a monoclonal antibody, a polyclonal antiserum, or a nucleic acid that reacts with the component of the objects of interest. Another probe with an enzyme, such as alkaline phosphatase or glucose oxidase then detects a reaction. The probe with the enzyme produces an enzymatic reaction that results in the objects of interest being stained a particular color. On the other hand, background areas and normal cells, for example, are stained colors, different from the particular color. Thus, the enzymatic reaction makes identifiable the objects of interest, if any, from the background areas and normal cells of the biological specimen.
A lab technician can manually examine the biological specimen to identify the objects of interest with a microscope. Recently, however, automated microscope systems and associated software for image analysis of the captured images of the slides have been developed to examine the biological specimen. These systems improve speed and accuracy in identifying the objects of interest in the biological specimen.
For example, U.S. Pat. No. 6,215,892 ('892 patent), assigned to ChromaVision Medical Systems, discloses an apparatus for automated cell analysis. The apparatus consists of a microscope with objective lenses, a stage for holding a slide, and a charged coupled device (CCD) camera. The slide includes the biological specimen to be examined. As a result, the CCD camera can capture an image of the biological specimen at a magnification level determined by the objective lenses.
The image of the biological specimen facilitates identification of the objects of interest. Picture elements, i.e., pixels, typically define the image of the biological specimen captured by the camera. Each of the pixels is made up of three components: a red component, a green component, and a blue components. Separate red, blue and green CCD cameras can be used to generate the red, blue and green pixels. The image of the biological specimen, i.e., the pixels defined by the red components, green components, and blue components, are transformed into a new representation or form. The new representation or form makes the objects of interest within the biological specimen readily identifiable.
The process of mathematically transforming an image from one representation, into another representation or form is known as applying a “color space transformation.” Several such color transformations exist, including hue saturation and intensity transformations, and a color transformation described in the '892 patent. The color space transformation described in the '892 patent involves forming a ratio of two different color components for each pixel in the image of the biological specimen. The ratio provides a means for discriminating color information. With three components for each pixel, nine possible color ratios can be formed: R/R, R/G, R/B, G/G, G/B, G/R, B/B, B/G, and B/R. The ratio to select for the color transformation depends on a range of colors expected in the biological specimen. For example, typical stains used for detecting objects of interest such as tumor cells are predominately red, as opposed to predominately green or blue. Thus, the pixels of an object of interest contain a red component which is larger than either the green or blue components. A ratio of red divided by blue (R/B) provides a value which is greater than one for tumor cells, but is approximately one for any clear or white areas on the slide. Since the remaining cells, i.e., normal cells, typically are stained blue, the R/B ratio for pixels of these latter cells yield values of less than one. The R/B ratio is preferred for clearly separating color information typical in these applications. Those pixels having color ratios that exceed a threshold level are associated with the objects of interest.
The automated cell analysis improves speed and accuracy in identifying the objects of interest in the biological specimen. The lab technician can manually review and evaluate whether pixels having ratios that exceed the threshold level are associated with the objects of interest. The lab technician need not manually analyze the biological specimen, as a whole, to identify the objects of interest.
It is not uncommon that areas of intense staining or foreign debris, such as dirt, appear on the slide. The areas of intense staining and foreign debris can cause the ratio-based color transformation of the '892 patent to improperly characterize the areas of intense staining or foreign debris as objects of interest. Since the components of the pixels in such areas are relatively “low,” and since low components that make up such pixels can appear in the denominator of the ratios, the color ratio can have falsely high values, causing “false” objects of interest to appear. These false objects of interest are objects identified as being objects of interest, but are in fact either normal cells or background areas of the biological specimen.
A presence of false objects of interest associated with the ratio-based color transformation drives one to use extensive morphological processing following the color transformation to recognize and attempt to eliminate the false objects of interest. Therefore, there exists a need for a method and system to more reliably identify objects of interest in the biological specimen, which is less sensitive to intense staining or foreign debris produced as a result of the staining process.